On Fri, 18 Mar 2016 11:25:48 +0000 Robin Murphy <robin.mur...@arm.com> wrote:
> On 18/03/16 09:30, Boris Brezillon wrote: > > On Thu, 17 Mar 2016 23:50:20 +0000 > > Russell King - ARM Linux <li...@arm.linux.org.uk> wrote: > > > >> On Thu, Mar 17, 2016 at 07:17:24PM -0400, ok...@codeaurora.org wrote: > >>> What is the correct way? I don't want to write engine->sram_dma = sram > >> > >> Well, what the driver _is_ wanting to do is to go from a CPU physical > >> address to a device DMA address. phys_to_dma() looks like the correct > >> thing there to me, but I guess that's just an offset and doesn't take > >> account of any IOMMU that may be in the way. > >> > >> If you have an IOMMU, then the whole phys_to_dma() thing is a total > >> failure as it only does a linear translation, and there are no > >> interfaces in the kernel to take account of an IOMMU in the way. So, > >> it needs something designed for the job, implemented and discussed by > >> the normal methods of proposing a new cross-arch interface for drivers > >> to use. > >> > >> What I'm certain of, though, is that the change proposed in this patch > >> will break current users of this driver: virt_to_page() on an address > >> returned by ioremap() is completely undefined, and will result in > >> either a kernel oops, or if not poking at memory which isn't a struct > >> page, ultimately resulting in something that isn't SRAM being pointed > >> to by "engine->sram_dma". > >> > > > > Or we could just do > > > > engine->sram_dma = res->start; > > > > which is pretty much what the SRAM/genalloc code is doing already. > > As Russell points out this is yet another type of "set up a DMA master > to access something other than kernel RAM" - there's already discussion > in progress over how to handle this for dmaengine slaves[1], so > gathering more use-cases might help distil exactly what the design of > not-strictly-DMA-but-so-closely-coupled-it-can't-really-live-anywhere-else > needs to be. > > Robin. > > [1]:http://lists.infradead.org/pipermail/linux-arm-kernel/2016-March/414422.html > Hm, interesting, thanks for the pointer. -- Boris Brezillon, Free Electrons Embedded Linux and Kernel engineering http://free-electrons.com -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
